Power transmission apparatus

ABSTRACT

During movement of a mass of a centrifugal clutch a radially inner position to a radially outer position and a resulting increase in torque transmitted from an input gear to an output shaft, a power transmission apparatus includes a first torque region where the apparatus restricts operation of a pressing assist cam and a second torque region where the apparatus allows operation of the pressing assist cam.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a power transmission apparatus that isable to freely transmit a rotational force of an input to an output orcut off the rotational force.

2. Description of the Related Art

Usually, a power transmission apparatus included in a motorcycle freelytransmits a driving force of an engine to a transmission and a drivingwheel or cuts off the driving force. Such a power transmission apparatusincludes an input member connected to an engine, an output memberconnected to a transmission and a driving wheel, a clutch memberconnected to the output member, and a pressure member that is able tomove close to or away from the clutch member. Bringing the pressuremember close to the clutch member presses driving and driven clutchplates against each other so as to transmit power therebetween. Bringingthe pressure member away from the clutch member releases a pressingforce exerted on the driving and driven clutch plates so as to cut offtransmission of the power therebetween.

As disclosed, for example, in WO 2013/183588, a power transmissionapparatus known in the art includes a centrifugal clutch means includinga weight member that moves from a radially inner position of a groove toa radially outer position thereof with centrifugal force produced byrotation of a clutch housing and is thus able to press driving anddriven clutch plates against each other. The power transmissionapparatus known in the art is able to apply centrifugal force to theweight member in accordance with the rotation of the clutch housingcaused by driving of an engine and is thus able to press the driving anddriven clutch plates against each other so as to transmit a drivingforce of the engine to a wheel.

The power transmission apparatus known in the art is provided with apressing assist cam to increase a pressing force exerted on the drivingand driven clutch plates when a rotational force received by an inputmember is transmittable to an output member. Accordingly, when a driverperforms a clutch operation involving causing the driving and drivenclutch plates to be pressed against each other, the power transmissionapparatus known in the art is able to reduce an operating force so as toenable smooth power transmission.

SUMMARY OF THE INVENTION

Unfortunately, during movement of the weight member of the centrifugalclutch means from the radially inner position to the radially outerposition and a resulting increase in torque transmitted from the inputmember to the output member, the pressing assist cam of the powertransmission apparatus known in the art may operate accidentally, forexample, when the driver rides a clutch. This may result in sudden,unintentional power transmission when a vehicle starts to move, makingit difficult for the vehicle to run smoothly.

Preferred embodiments of the present invention provide powertransmission apparatuses that are each able to prevent sudden, untimelypower transmission caused by accidental operation of a pressing assistcam when a vehicle including a centrifugal clutch starts to move.

A power transmission apparatus according to a preferred embodiment ofthe present invention includes a clutch housing that rotates togetherwith an input that rotates due to a driving force of an engine of avehicle, the clutch housing including a plurality of driving clutchplates attached thereto, a clutch including a plurality of driven clutchplates attached thereto, the driven clutch plates being alternatelyarranged with the driving clutch plates attached to the clutch housing,the clutch being connected to an output that is able to rotate a wheelof the vehicle, a pressure applicator movable between an operatingposition where the driving and driven clutch plates are pressed againsteach other so as to enable transmission of the driving force of theengine to the wheel and a non-operating position where a pressing forceexerted on the driving and driven clutch plates is released so as to cutoff transmission of the driving force of the engine to the wheel, acentrifugal clutch including a mass movable from a radially innerposition to a radially outer position with centrifugal force produced byrotation of the clutch housing, the centrifugal clutch being configuredto, when the mass is located at the radially outer position, press thedriving and driven clutch plates against each other so as to enabletransmission of the driving force of the engine to the wheel andconfigured to, when the mass is located at the radially inner position,release the pressing force exerted on the driving and driven clutchplates so as to cut off transmission of the driving force of the engineto the wheel, and a pressing assist cam to increase the pressing forceexerted on the driving and driven clutch plates when a rotational forcereceived by the input is transmittable to the output. During movement ofthe mass of the centrifugal clutch from the radially inner position tothe radially outer position and a resulting increase in torquetransmitted from the input to the output, the power transmissionapparatus includes a first torque region where the apparatus restrictsoperation of the pressing assist cam and a second torque region wherethe apparatus allows operation of the pressing assist cam.

The clutch may include a first clutch connected to the output, and asecond clutch having the driven clutch plates attached thereto. Thepressing assist cam includes an inclined surface of the first clutch andan inclined surface of the pressure applicator that face each other.

The power transmission apparatus may be configured to, in the firsttorque region, move the first clutch and the pressure applicator intoabutment with each other so as to restrict operation of the pressingassist cam, and configured to, in the second torque region, move thefirst clutch and the pressure applicator away from each other so as toallow operation of the pressing assist cam.

The centrifugal clutch may be configured to, in the first torque region,move the second clutch such that the first clutch and the pressureapplicator are kept in abutment with each other, and configured to, inthe second torque region, move the second clutch and the pressureapplicator such that the first clutch and the pressure applicator arespaced away from each other.

In the power transmission apparatus, a transition may be made from thefirst torque region to the second torque region in course of operationof the centrifugal clutch.

According to a preferred embodiment of the present invention, duringmovement of the mass of the centrifugal clutch from the radially innerposition to the radially outer position and a resulting increase intorque transmitted from the input to the output, the power transmissionapparatus includes the first torque region where the apparatus restrictsoperation of the pressing assist cam, and the second torque region wherethe apparatus allows operation of the pressing assist cam. Consequently,this preferred embodiment is able to prevent sudden, untimely powertransmission caused by accidental operation of the pressing assist camwhen the vehicle including the centrifugal clutch starts to move.

According to another preferred embodiment of the present invention, theclutch includes the first clutch connected to the output, and the secondclutch having the driven clutch plates attached thereto. The pressingassist cam includes the inclined surface of the first clutch and theinclined surface of the pressure applicator that face each other.Consequently, with the first clutch and the pressure applicator, thispreferred embodiment enables operation of the pressing assist cam.

According to a further preferred embodiment of the present invention,the power transmission apparatus is configured to, in the first torqueregion, move the first clutch and the pressure applicator into abutmentwith each other so as to restrict operation of the pressing assist cam,and is configured to, in the second torque region, move the first clutchand the pressure applicator away from each other so as to allowoperation of the pressing assist cam. Consequently, this preferredembodiment is able to accurately and smoothly restrict operation of thepressing assist cam in the first torque region and allow operation ofthe pressing assist cam in the second torque region.

According to an additional preferred embodiment of the presentinvention, the centrifugal clutch is configured to, in the first torqueregion, move the second clutch such that the first clutch and thepressure applicator are kept in abutment with each other, and isconfigured to, in the second torque region, move the second clutch andthe pressure applicator such that the first clutch and the pressureapplicator are spaced away from each other. Consequently, this preferredembodiment is able to restrict operation of the pressing assist cam inthe first torque region and allow operation of the pressing assist camin the second torque region by operating the centrifugal clutch.

According to another preferred embodiment of the present invention, atransition is made from the first torque region to the second torqueregion in the course of operation of the centrifugal clutch.Consequently, this preferred embodiment is able to continuously andsmoothly restrict operation of the pressing assist cam in the firsttorque region and allow operation of the pressing assist cam in thesecond torque region.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of a power transmission apparatus accordingto a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1 .

FIG. 3 is a cross-sectional view taken along the line in FIG. 1 .

FIG. 4 is a perspective view of a clutch housing of the powertransmission apparatus.

FIG. 5 is a three-view drawing of a first clutch member of the powertransmission apparatus.

FIG. 6 is a perspective view of the first clutch member.

FIG. 7 is a three-view drawing of a second clutch member of the powertransmission apparatus.

FIG. 8 is a perspective view of the second clutch member.

FIG. 9 is a three-view drawing of a pressure member of the powertransmission apparatus.

FIG. 10 is a perspective view of the pressure member.

FIG. 11 is a longitudinal cross-sectional view of a centrifugal clutchof the power transmission apparatus.

FIG. 12 is a partially cutaway perspective view of the centrifugalclutch.

FIG. 13 is a three-view drawing of a holding member included in thecentrifugal clutch.

FIG. 14 is a three-view drawing of a supporting member included in thecentrifugal clutch.

FIG. 15 is a three-view drawing of a pressing member included in thecentrifugal clutch.

FIG. 16 is a four-view drawing of a weight member included in thecentrifugal clutch.

FIG. 17 is a cross-sectional view taken along the line XVII-XVII in FIG.16 .

FIG. 18 is a plan view illustrating a state of the centrifugal clutchwhere each weight member is located at a radially inner position.

FIG. 19 is a plan view illustrating a state of the centrifugal clutchwhere each weight member is located at a radially outer position.

FIG. 20A is a schematic diagram describing how a pressing assist cam ofthe power transmission apparatus acts.

FIG. 20B is a schematic diagram describing how a back torque limiter camof the power transmission apparatus acts.

FIG. 21 is a schematic diagram of a vehicle in which the powertransmission apparatus is used.

FIG. 22 is a cross-sectional view illustrating a state of the powertransmission apparatus where each weight member is located at theradially inner position.

FIG. 23 is a cross-sectional view illustrating a state of the powertransmission apparatus where each weight member is located at anintermediate position between the radially inner position and theradially outer position.

FIG. 24 is a cross-sectional view illustrating a state of the powertransmission apparatus where each weight member is located at theradially outer position.

FIG. 25 is a cross-sectional view illustrating a state of the powertransmission apparatus where each weight member is located at theradially outer position and the pressure member is located at anon-operating position.

FIG. 26 is a graph illustrating relationships between engine rotationspeeds and torques/assist cam positions for the power transmissionapparatus according to the present preferred embodiment.

FIG. 27 is a graph illustrating relationships between engine rotationspeeds and torques/assist cam positions for a power transmissionapparatus known in the art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowin detail with reference to the drawings. As illustrated in FIG. 21 , apower transmission apparatus K according to the present preferredembodiment is disposed in a vehicle so as to freely transmit a drivingforce of an engine E to a driving wheel T through a transmission M orcut off the driving force. As illustrated in FIGS. 1 to 17 , the powertransmission apparatus K includes a clutch housing 2 provided with aninput gear 1 (which is an input) that rotates with the driving force ofthe engine E of the vehicle, an output shaft 3 (which is an output)connected to the transmission M, a clutch (which includes a first clutchmember 4 a and a second clutch member 4 b); a pressure member 5 (whichis a pressure applicator); a plurality of driving clutch plates 6; aplurality of driven clutch plates 7; a centrifugal clutch 9 includingweight members 10 (which are masses); and an auxiliary clutch plate 17.

The input gear 1 is rotatable around the output shaft 3 upon receiving adriving force (or a rotational force) transmitted from the engine E. Theinput gear 1 is connected to the clutch housing 2 with a fastener, suchas a rivet. The clutch housing 2 is a cylindrical member with an openingdefined at its right end in FIGS. 2 and 3 and is connected to the inputgear 1. The clutch housing 2 is rotatable together with rotation of theinput gear 1 with the driving force of the engine E.

As illustrated in FIG. 4 , the clutch housing 2 is circumferentiallyprovided with a plurality of cut-outs 2 a. The driving clutch plates 6are fitted to the cut-outs 2A and thus attached to the clutch housing 2.The driving clutch plates 6 are each made of a substantially annularplate material. The driving clutch plates 6 are rotatable together withrotation of the clutch housing 2. The driving clutch plates 6 areslidable in an axial direction (which corresponds to a right-leftdirection in FIGS. 2 and 3 ).

The driven clutch plates 7 are attached to the clutch (which includesthe first clutch member 4 a and the second clutch member 4 b). Thedriven clutch plates 7 are arranged alternately with the driving clutchplates 6 attached to the clutch housing 2. The clutch is connected tothe output shaft 3 (i.e., the output) that is able to rotate the drivingwheel T through the transmission M of the vehicle. The clutch isprovided by assembling two members, i.e., the first clutch member 4 aand the second clutch member 4 b, to each other.

The output shaft 3 is inserted through an insertion hole (see FIGS. 5and 6 ) defined in the center of the first clutch member 4 a. A gearprovided on the first clutch member 4 a and a gear provided on theoutput shaft 3 mesh with each other so as to be connected to each otherin the direction of rotation. As illustrated in FIGS. 5 and 6 , thefirst clutch member 4 a is provided with inclined surfaces 4 aa eachdefining a pressing assist cam, and inclined surfaces 4 ab each defininga back torque limiter cam. The reference signs “4 ac” in FIGS. 5 and 6each indicate a boss provided with an insertion hole for a bolt B forconnection between the first clutch member 4 a and a securing member 8.

As illustrated in FIGS. 7 and 8 , the second clutch member 4 b is anannular member provided with a flange 4 bb. The driven clutch plates 7are spline-fitted to a spline-fitting portion 4 ba provided on the outerperipheral surface of the second clutch member 4 b and are thus attachedto the second clutch member 4 b. As illustrated in FIGS. 2 and 3 , thepressure member 5 is assembled to the clutch (which includes the firstclutch member 4 a and the second clutch member 4 b) such that thedriving and driven clutch plates 6 and 7 are alternately stacked andsecured between a flange 5 c of the pressure member 5 and the flange 4bb of the second clutch member 4 b.

As illustrated in FIGS. 9 and 10 , the pressure member 5 is a diskmember provided with the flange 5 c extending along the peripheral edgeof the pressure member 5. The pressure member 5 is movable between anoperating position where the driving and driven clutch plates 6 and 7are pressed against each other so as to enable transmission of thedriving force of the engine E to the wheel, and a non-operating positionwhere a pressing force exerted on the driving and driven clutch plates 6and 7 is released so as to cut off transmission of the driving force ofthe engine E to the wheel.

More specifically, as illustrated in FIGS. 7 and 8 , the spline-fittingportion 4 ba provided on the second clutch member 4 b defines integralprojections and depressions along substantially the entire outerperipheral side surface of the second clutch member 4 b. Fitting thedriven clutch plates 7 to recessed grooves that define thespline-fitting portion 4 ba restricts movement of the driven clutchplates 7 in the direction of rotation while allowing movement of thedriven clutch plates 7 in the axial direction relative to the secondclutch member 4 b. The driven clutch plates 7 are rotatable togetherwith the second clutch member 4 b.

The driven clutch plates 7 are stacked alternately with the drivingclutch plates 6, making it possible to allow the clutch plates 6 and 7adjacent to each other to be pressed against each other or release thepressing force exerted thereon. In other words, the clutch plates 6 and7 are allowed to slide in the axial direction of the second clutchmember 4 b. Engaging a clutch by pressing the clutch plates (6 a, 6 b, 7a, 7 b) against each other enables a rotational force of the clutchhousing 2 to be transmitted to the output shaft 3 through the secondclutch member 4 b and the first clutch member 4 a. Disengaging theclutch by releasing the pressing force exerted on the clutch plates (6a, 6 b, 7 a, 7 b) causes the first clutch member 4 a and the secondclutch member 4 b to stop following the rotation of the clutch housing 2such that no rotational force will be transmitted to the output shaft 3.

Accordingly, a state where the driving and driven clutch plates 6 and 7are pressed against each other enables transmission of a rotationalforce (i.e., the driving force of the engine E), which is received bythe clutch housing 2, to the driving wheel (or the transmission M)through the output shaft 3 (i.e., the output), and a state where thedriving and driven clutch plates 6 and 7 are not pressed against eachother makes it possible to cut off transmission of the rotational force(or the driving force of the engine E), which is received by the clutchhousing 2, to the output shaft 3 (i.e., the output).

As illustrated in FIGS. 9 and 10 , the pressure member 5 iscircumferentially provided with a plurality of fitting holes 5 d (i.e.,three fitting holes 5 d in the present preferred embodiment). A clutchspring S is fitted into each of the fitting holes 5 d. As illustrated inFIG. 2 , one end of each clutch spring S housed in the associatedfitting hole 5 d is in abutment with the securing member 8, such thateach clutch spring S urges the pressure member 5 in a direction in whichthe driving and driven clutch plates 6 and 7 are to be pressed againsteach other. Operating a clutch actuator (not illustrated) makes itpossible to press the driving and driven clutch plates 6 and 7 againsteach other or stop pressing the driving and driven clutch plates 6 and 7against each other.

In the present preferred embodiment, as illustrated in FIGS. 5, 6, 9,and 10 , the first clutch member 4 a is provided with the inclinedsurfaces 4 aa and 4 ab, and the pressure member 5 is provided withinclined surfaces 5 a and 5 b respectively facing the inclined surfaces4 aa and 4 ab. Specifically, each inclined surface 4 aa comes intoabutment with the associated inclined surface 5 a so as to provide thepressing assist cam, and each inclined surface 4 ab comes into abutmentwith the associated inclined surface 5 b so as to provide the backtorque limiter cam.

Suppose that the rotation speed of the engine E is increased and theresulting rotational force received by the input gear 1 and the clutchhousing 2 is transmittable to the output shaft 3 through the firstclutch member 4 a and the second clutch member 4 b (which means that theweight members 10 are each located at a radially outer position). Inthis case, as illustrated in FIG. 20A, the rotational force is appliedto the pressure member 5 in an a-direction, which exerts a force on thepressure member 5 in a c-direction in FIG. 20A under the action of eachpressing assist cam. The pressure member 5 is thus moved in a directionin which its flange 5 c comes closer to the flange 4 bb of the secondclutch member 4 b (i.e., leftward in FIGS. 2 and 3 ) so as to increasethe pressing force exerted on the driving and driven clutch plates 6 and7.

Suppose that the rotation of the output shaft 3 exceeds the rotationspeed of the input gear 1 and the clutch housing 2, resulting in a backtorque. In this case, as illustrated in FIG. 20B, a rotational force isapplied to the clutch member 4 in a b-direction, which moves thepressure member 5 in a d-direction in FIG. 20B under the action of eachback torque limiter cam so as to release the pressing force exerted onthe driving and driven clutch plates 6 and 7. This makes it possible toprevent a malfunction in the power transmission apparatus K or a powersource (i.e., the engine E) caused by the back torque.

As illustrated in FIGS. 11 to 19 , the centrifugal clutch 9 includes theweight members 10 each movable from a radially inner position (see FIG.18 ) to the radially outer position (see FIG. 19 ) with centrifugalforce produced by rotation of the clutch housing 2. When the weightmembers 10 are each located at the radially outer position, thecentrifugal clutch 9 presses the driving and driven clutch plates 6 and7 against each other so as to enable transmission of the driving forceof the engine E to the wheel (i.e., the driving wheel T). When theweight members 10 are each located at the radially inner position, thecentrifugal clutch 9 releases the pressing force exerted on the drivingand driven clutch plates 6 and 7 so as to cut off transmission of thedriving force of the engine E to the wheel (i.e., the driving wheel T).

Specifically, the centrifugal clutch 9 includes the weight members 10,each of which is a substantially symmetrical polygonal member, a holdingmember 11 having a supporting member 13 attached thereto, a pressingmember 12, first spherical members 14, second spherical members 15, andurging members 16, each of which is a coil spring. The holding member 11and the pressing member 12 are each circumferentially provided with aplurality of protrusions. Similarly to the driving clutch plates 6, theprotrusions are fitted to the cut-outs 2 a of the clutch housing 2 suchthat the holding member 11 and the pressing member 12 are attached tothe clutch housing 2. Accordingly, the holding member 11 and thepressing member 12 are each movable in the axial direction of the clutchhousing 2 and in engagement with the clutch housing 2 in the directionof rotation so as to be rotatable together with the clutch housing 2.

As illustrated in FIG. 16 , each weight member 10 is a substantiallysymmetrical polygonal member including a first surface X and a secondsurface Y. As illustrated in FIGS. 16 and 17 , each weight member 10includes through holes 10 a passing through the weight member 10 fromthe first surface X to the second surface Y, insertion portions 10 bdefined in the second surface Y, and a groove 10 c defined in the firstsurface X. As illustrated in FIGS. 18 and 19 , the weight members 10 areeach housed in an associated one of housing portions 11 a of the holdingmember 11. When no centrifugal force is applied to the weight members10, the weight members 10 are each held at the radially inner position(see FIG. 18 ). Application of centrifugal force to the weight members10 moves the weight members 10 outward against the urging force of theurging members 16 such that each weight member 10 reaches the radiallyouter position (see FIG. 19 ).

The holding member 11 holds the weight members 10 such that each weightmember 10 is movable between the radially inner position and theradially outer position. As illustrated in FIG. 13 , the holding member11 is an annular member. The holding member 11 includes the housingportions 11 a which are arranged in the circumferential direction of theholding member 11 and in which the weight members 10 are to be housed,groove geometries 11 b defined in the housing portions 11 a, and apushing surface 11 c. Each housing portion 11 a has a recessed shapeconforming to the shape and moving range of the associated weight member10. An inner peripheral wall surface 11 aa of each housing portion 11 ais allowed to abut against first ends of the associated urging members16.

The supporting member 13 is secured to a surface of the holding member11, which is provided with the housing portions 11 a. As illustrated inFIG. 14 , the supporting member 13 is provided with radially extendingholding portions 13 a. The holding portions 13 a each conform to thegroove 10 c of the associated weight member 10, so that the weightmembers 10 are held by the holding member 11. Specifically, the firstsurface X of each weight member 10 is centrally provided with the groove10 c extending in a direction from the radially inner position to theradially outer position. Causing each holding portion 13 a to conform tothe associated groove 10 c allows the holding member 11 to hold theweight members 10 such that each weight member 10 is movable in a radialdirection (i.e., in a direction from the radially inner position to theradially outer position).

Movement of each weight member 10 from the radially inner position tothe radially outer position causes the pressing member 12 to move in astacking direction of the driving and driven clutch plates 6 and 7(i.e., rightward in FIGS. 2 and 3 ) such that the driving and drivenclutch plates 6 and 7 are pressed against each other. Specifically, asillustrated in FIG. 15 , the pressing member 12 is an annular member.The pressing member 12 includes inclined grooves 12 a arranged in thecircumferential direction of the pressing member 12, groove geometries12 b each defined at a position where the associated inclined groove 12a is defined, and a pushing surface 12 c.

Each inclined groove 12 a is defined at a position corresponding to theposition of the associated weight member 10. Each inclined groove 12 ais inclined upward from its inner portion to its outer portion. Thus,when the clutch housing 2 is stationary, each weight member 10 is heldat the radially inner position with the urging force of the associatedurging members 16. Rotation of the clutch housing 2 applies centrifugalforce to the weight members 10 so as to move the weight members 10 alongthe inclined grooves 12 a inclined upward. This moves the pressingmember 12 away from the holding member 11 (i.e., in the direction inwhich the driving and driven clutch plates 6 and 7 are to be pressedagainst each other).

When the holding member 11 and the pressing member 12 are assembled toeach other, with the weight members 10 interposed therebetween, eachinclined groove 12 a is located at a position corresponding to theposition of the associated weight member 10 as illustrated in FIGS. 11and 12 . With centrifugal force, the weight members 10 each move alongthe inclined grooves 12 a from the radially inner position to theradially outer position, which moves the pressing member 12 in adirection indicated by the arrow in FIG. 11 (i.e., rightward in FIG. 11). The pushing surface 12 c of the pressing member 12 thus pushes thedriving and driven clutch plates 6 and 7 such that the driving anddriven clutch plates 6 and 7 are pressed against each other. Theresulting reaction force moves the holding member 11 in a directionopposite to the direction indicated by the arrow in FIG. 11 (i.e.,leftward in FIG. 11 ), so that the pushing surface 11 c of the holdingmember 11 presses the auxiliary clutch plate 17.

As illustrated in FIGS. 18 and 19 , the weight members 10 according tothe present preferred embodiment are each housed in an associated one ofthe housing portions 11 a arranged in the circumferential direction ofthe holding member 11 such that the weight members 10 are movableradially. More than one urging member 16 (i.e., two urging members 16 inthe present preferred embodiment) is disposed in the circumferentialdirection between the inner peripheral wall surface 11 aa (see FIG. 13 )of each of the housing portions 11 a and an associated one of the weightmembers 10 so as to urge the associated weight member 10 from theradially outer position to the radially inner position. In thispreferred embodiment, the inner peripheral wall surface 11 aa of eachhousing portion 11 a is a flat surface in abutment with the first endsof the associated urging members 16 such that the urging members 16 arestably securable.

The weight members 10 according to the present preferred embodiment areeach provided with the tunnel-shaped insertion portions 10 b which areopenings defined in a surface of each weight member 10 facing theholding member 11 (i.e., the second surface Y in FIG. 17 ) and intowhich the urging members 16 are inserted such that the urging members 16are attachable to the associated weight member 10. The weight members10, with the urging members 16 inserted into the insertion portions 10b, are housed in the housing portions 11 a of the holding member 11. Theurging members 16 are thus secured such that each urging member 16 isinterposed between the inner peripheral wall surface 11 aa of theassociated housing portion 11 a and the associated weight member 10. Theurging members 16 are disposed such that the first end of each urgingmember 16 is in abutment with the associated inner peripheral wallsurface 11 aa and a second end of each urging member 16 is in abutmentwith an end wall surface 10 ba of the associated insertion portion 10 b,making it possible to urge each weight member 10 from the radially outerposition to the radially inner position.

Each first spherical member 14 is a steel ball attached to theassociated weight member 10. As illustrated in FIGS. 16 and 17 , eachfirst spherical member 14 is partially protruded from a first opening 10aa (i.e., a small-diameter opening in the first surface X) of thethrough hole 10 a defined in the associated weight member 10 and is incontact with a rolling contact surface of the pressing member 12 so asto be rollable thereon. Each second spherical member 15 is a steel ballattached to the associated weight member 10. As illustrated in FIGS. 16and 17 , each second spherical member 15 is partially protruded from asecond opening 10 ab (i.e., a large-diameter opening in the secondsurface Y) of the through hole 10 a defined in the associated weightmember 10 and is in contact with a rolling contact surface of theholding member 11 so as to be rollable thereon.

As illustrated in FIG. 17 , the through holes 10 a according to thepresent preferred embodiment are tapered such that the diameter of eachthrough hole 10 a continuously increases from the first opening 10 aa(i.e., the small-diameter opening in the first surface X) to the secondopening 10 ab (i.e., the large-diameter opening in the second surfaceY). Disconnection of each first spherical member 14 from the associatedthrough hole 10 a is prevented by the outer peripheral edge of one ofthe first opening 10 aa and the second opening 10 ab that has a smallerdiameter (which is, in the present preferred embodiment, the firstopening 10 aa in the first surface X). The first and second sphericalmembers 14 and 15 according to the present preferred embodiment arespherical members having different diameters in accordance with theinner diameters of the through holes 10 a. The second spherical members15 are larger in diameter than the first spherical members 14. Thesmall-diameter first spherical members 14 are each rollable while beingin contact with the inner peripheral surface of the associated throughhole 10 a, and at the same time, disconnection of the small-diameterfirst spherical members 14 is prevented by the small-diameter openingedges of the through holes 10 a.

As illustrated in FIGS. 11 and 12 , disconnection of the secondspherical members 15 is prevented by the rolling contact surface of theholding member 11. Thus, disconnection of the small-diameter firstspherical members 14 is prevented by the small-diameter opening edges ofthe through holes 10 a, and disconnection of the large-diameter secondspherical members 15 is prevented by the rolling contact surface of theholding member 11 while the second spherical members 15 are partiallyprotruded from the large-diameter openings of the through holes 10 a. Inthe present preferred embodiment, the large-diameter second sphericalmembers 15 are assembled to the weight members 10 such that the secondspherical members 15 face the rolling contact surface of the holdingmember 11. Alternatively, the second spherical members 15 may beassembled to the weight members 10 such that the second sphericalmembers 15 face the rolling contact surface of the pressing member 12.In this case, disconnection of the small-diameter first sphericalmembers 14 is prevented by the small-diameter opening edges of thethrough holes 10 a, and disconnection of the large-diameter secondspherical members 15 is prevented by the rolling contact surface of thepressing member 12 while the second spherical members 15 are partiallyprotruded from the large-diameter openings of the through holes 10 a.

As illustrated in FIG. 13 , the rolling contact surface of the holdingmember 11 (which is, in the present preferred embodiment, a surface ofthe holding member 11 on which the second spherical members 15 roll)includes the groove geometries 11 b each extending in the direction ofmovement of the associated weight member 10 (i.e., a directionconnecting the radially inner position to the radially outer position).As illustrated in FIG. 15 , the rolling contact surface of the pressingmember 12 (which is, in the present preferred embodiment, a surface ofthe pressing member 12 on which the first spherical members 14 roll)includes the groove geometries 12 b each extending in the direction ofmovement of the associated weight member 10 (i.e., a directionconnecting the radially inner position to the radially outer position).

As illustrated in FIGS. 16, 18, and 19 , the first and second sphericalmembers 14 and 15 according to the present preferred embodiment areprovided such that more than one first spherical member 14 and more thanone second spherical member 15 are arranged in the circumferentialdirection of the holding member 11 (i.e., such that two first sphericalmembers 14 and two second spherical members 15 are arranged in the widthdirection of each weight member 10 in the present preferred embodiment).In response to movement of the weight members 10, the first sphericalmembers 14 are movable along the groove geometries 12 b while rollingwithin the through holes 10 a, and the second spherical members 15 aremovable along the groove geometries 11 b while rolling within thethrough holes 10 a.

The auxiliary clutch plate 17 is an annular member different in diameterfrom the driving and driven clutch plates 6 and 7 (i.e., smaller indiameter than the driving and driven clutch plates 6 and 7 in thepresent preferred embodiment). As illustrated in FIGS. 2 and 3 , theoutput shaft 3 (i.e., the output) is inserted through a central opening17 a of the auxiliary clutch plate 17 so as to be fitted thereto. Theauxiliary clutch plate 17 includes a pushed surface 17 b facing thepushing surface 11 c of the holding member 11.

When the weight members 10 are each located at the radially outerposition (i.e., when the driving and driven clutch plates 6 and 7 arepressed against each other), the auxiliary clutch plate 17 is able totransmit the driving force of the engine E to the output shaft 3 uponbeing pushed by the pushing surface 11 c of the holding member 11 andpressed against the pushing surface 11 c. When the weight members 10 areeach located at the radially inner position (i.e., when the pressingforce exerted on the driving and driven clutch plates 6 and 7 isreleased), the auxiliary clutch plate 17 is able to cut off transmissionof the driving force of the engine E to the output shaft 3 upon beingrelieved of a pressing force applied thereto, owing to a decrease inpushing force exerted by the pushing surface 11 c of the holding member11.

Upon movement of each weight member 10 to the radially outer position,each inclined groove 12 a functions as a cam so as to cause the holdingmember 11 and the pressing member 12 to move away from each other.Accordingly, the pushing surface 12 c of the pressing member 12 pressesthe driving and driven clutch plates 6 and 7 against each other, and thepushing surface 11 c of the holding member 11 pushes the pushed surface17 b of the auxiliary clutch plate 17 such that the pushed surface 17 bis pressed against the pushing surface 11 c so as to transmit thedriving force of the engine E to the driving wheel T.

In the power transmission apparatus K according to the present preferredembodiment, the through holes 10 a of the weight members 10 included inthe centrifugal clutch 9 each have a tapered shape from the firstopening 10 aa to the second opening 10 ab. Disconnection of each firstspherical member 14 is prevented by the outer peripheral edge of one ofthe associated first opening 10 aa and the associated second opening 10ab that has a smaller diameter. Accordingly, the present preferredembodiment is able to easily and accurately attach the first sphericalmembers 14 to the weight members 10, resulting in a reduction inmanufacturing cost.

The first and second spherical members 14 and 15 are spherical membershaving different diameters in accordance with the inner diameters of thethrough holes 10 a. The first and second spherical members 14 and 15 arerollable while being in contact with the inner peripheral surfaces ofthe through holes 10 a. Thus, during movement of the weight members 10,the first and second spherical members 14 and 15 are rollable in astable manner such that the weight members 10 move smoothly.Disconnection of the second spherical members 15 according to thepresent preferred embodiment is prevented by the rolling contact surfaceof the holding member 11 or the pressing member 12. Consequently, thepresent preferred embodiment is able to facilitate preventingdisconnection of the first and second spherical members 14 and 15.

The rolling contact surface of the holding member 11 or the pressingmember 12 includes the groove geometries (11 b, 12 b) extending in thedirection of movement of the weight members 10. Accordingly, the presentpreferred embodiment enables smoother movement of the weight members 10while reliably preventing disconnection of the second spherical members15 from the large-diameter openings and disconnection of the firstspherical members 14 from the small-diameter openings.

In addition, the weight members 10 according to the present preferredembodiment are each housed in an associated one of the housing portions11 a arranged in the circumferential direction of the holding member 11and are thus movable radially. More than one urging member 16 isdisposed in the circumferential direction between the inner peripheralwall surface 11 aa of each of the housing portions 11 a and anassociated one of the weight members 10 so as to urge the associatedweight member 10 from the radially outer position to the radially innerposition. Accordingly, the present preferred embodiment is able toaccurately urge each weight member 10 from the radially outer positionto the radially inner position, enabling stable movement of the weightmembers 10 in accordance with centrifugal force.

The weight members 10 according to the present preferred embodiment areeach provided with the insertion portions 10 b which are openingsdefined in the surface of each weight member 10 facing the holdingmember 11 and into which the urging members 16 are inserted such thatthe urging members 16 are attachable to the associated weight member 10.This facilitates assembling the urging members 16 to the weight members10. The weight members 10 according to the present preferred embodimentare each provided with the groove 10 c extending in a direction from theradially inner position to the radially outer position. The holdingmember 11 (or specifically, the supporting member 13 secured to theholding member 11 so as to be integral therewith) is provided with theholding portions 13 a each conforming to the associated groove 10 c andholding the associated weight member 10. Accordingly, the presentpreferred embodiment enables stable movement of the weight members 10.

The centrifugal clutch 9 according to the present preferred embodimentincludes the first spherical members 14 that are partially protrudedfrom the first openings 10 aa of the through holes 10 a defined in theweight members 10 and are in contact with the rolling contact surface(or the groove geometries 12 b) of the pressing member 12 so as to berollable thereon, and the second spherical members 15 that are partiallyprotruded from the second openings 10 ab of the through holes 10 adefined in the weight members 10 and are in contact with the rollingcontact surface (or the groove geometries 11 b) of the holding member 11so as to be rollable thereon. Accordingly, the present preferredembodiment enables more stable movement of the weight members 10.

The holding member 11 or the pressing member 12, in particular, includesthe groove geometries (11 b, 12 b) extending in the direction ofmovement of the weight members 10. The groove geometries (11 b, 12 b)define the rolling contact surface for the first spherical members 14 orthe second spherical members 15 so as to enable smoother movement of theweight members 10. The first and second spherical members 14 and 15according to the present preferred embodiment are provided such thatmore than one first spherical member 14 and more than one secondspherical member 15 are arranged in the circumferential direction of theholding member 11 (i.e., the width direction of each weight member 10).Accordingly, the present preferred embodiment enables more stablemovement of the weight members 10.

In the course of movement of each weight member 10 of the centrifugalclutch 9 from the radially inner position to the radially outer positionand a resulting increase in torque transmitted from the input gear 1(i.e., the input) to the output shaft 3 (i.e., the output), the powertransmission apparatus K according to the present preferred embodimentincludes, as illustrated in FIG. 26 , a first torque region α1 where theapparatus restricts operation of the pressing assist cams (which arecams including the inclined surfaces 4 aa and 5 a) and a second torqueregion α2 where the apparatus allows operation of the pressing assistcams.

Specifically, as illustrated in FIGS. 5 and 6 , a portion of a surfaceof the first clutch member 4 a according to the present preferredembodiment, which faces the pressure member 5, defines an abutmentsurface 4 ad. As illustrated in FIGS. 9 and 10 , a portion of a surfaceof the pressure member 5, which faces the first clutch member 4 a,defines an abutment surface 5 e. With the first clutch member 4 a, thesecond clutch member 4 b, and the pressure member 5 assembled to eachother (and with no torque being transmitted from the input gear 1 (i.e.,the input) to the output shaft 3 (i.e., the output)), the abutmentsurface 4 ad and the abutment surface 5 e are in abutment with eachother as illustrated in FIGS. 2 and 3 .

With the abutment surface 4 ad and the abutment surface 5 e in abutmentwith each other as mentioned above (i.e., when the apparatus is in thefirst torque region α1 corresponding to a time period between a time t1and a time t2 in FIG. 26 ), in the course of movement of each weightmember 10 of the centrifugal clutch 9 from the radially inner position(see FIG. 22 ) to an intermediate position (see FIG. 23 ) and aresulting increase in torque transmitted from the input gear 1 (i.e.,the input) to the output shaft 3 (i.e., the output), the first clutchmember 4 a and the pressure member 5 are not allowed to move relative toeach other, which restricts operation of the pressing assist cams.

Each weight member 10 of the centrifugal clutch 9 then further movesfrom the intermediate position (see FIG. 23 ) to the radially outerposition (see FIG. 24 ), and the driving and driven clutch plates 6 and7 are pushed by the flange 4 bb of the second clutch member 4 b and thuspressed against each other. When the pushing force of the flange 4 bb isequal to or greater than the urging force of the clutch springs S, thesecond clutch member 4 b and the pressure member 5 are moved in theaxial direction (i.e., rightward in FIGS. 2 and 3 ) relative to thefirst clutch member 4 a, so that the abutment surface 4 ad of the firstclutch member 4 a and the abutment surface 5 e of the pressure member 5are spaced away from each other. FIG. 25 illustrates a state in whichthe weight members 10 are each located at the radially outer positionand the pressure member 5 is located at the non-operating position(i.e., a clutch-disengaged state).

With the abutment surface 4 ad and the abutment surface 5 e spaced awayfrom each other as mentioned above (i.e., when the apparatus is in thesecond torque region α2 corresponding to a time period subsequent to thetime t2 in FIG. 26 ), in the course of movement of each weight member 10of the centrifugal clutch 9 from the radially inner position to theradially outer position and a resulting increase in torque transmittedfrom the input gear 1 (i.e., the input) to the output shaft 3 (i.e., theoutput), the first clutch member 4 a and the pressure member 5 areallowed to move relative to each other, which allows operation of thepressing assist cams.

In other words, the present preferred embodiment involves, in the firsttorque region α1, moving the abutment surface 4 ad of the first clutchmember 4 a and the abutment surface 5 e of the pressure member 5 intoabutment with each other so as to restrict operation of the pressingassist cams, and involves, in the second torque region α2, moving theabutment surface 4 ad of the first clutch member 4 a and the abutmentsurface 5 e of the pressure member 5 away from each other so as to allowoperation of the pressing assist cams.

In the present preferred embodiment, the centrifugal clutch 9 isconfigured to, in the first torque region α1, move the second clutchmember 4 b without moving the first clutch member 4 a such that thefirst clutch member 4 a and the pressure member 5 are kept in abutmentwith each other, and configured to, in the second torque region α2, movethe second clutch member 4 b and the pressure member 5 such that theabutment surface 4 ad of the first clutch member 4 a and the abutmentsurface 5 e of the pressure member 5 are spaced away from each other.The present preferred embodiment, in particular, involves making atransition from the first torque region α1 to the second torque regionα2 in the course of operation of the centrifugal clutch 9.

For example, suppose that a power transmission apparatus known in theart includes a first clutch member 4 a and a pressure member 5 that donot come into abutment with each other and includes no first torqueregion α1 (but includes only a torque region β). In this case, asillustrated in FIG. 27 , pressing assist cams unfortunately operate upongeneration of a torque transmitted from an input gear 1 (i.e., an input)to an output shaft 3 (i.e., an output), which is caused by movement ofeach weight member 10 of a centrifugal clutch 9 from a radially innerposition to a radially outer position. This results in sudden,unintentional power transmission when a vehicle starts to move, makingit difficult for the vehicle to run smoothly.

In the course of movement of each weight member 10 of the centrifugalclutch 9 from the radially inner position to the radially outer positionand a resulting increase in torque transmitted from the input gear 1(i.e., the input) to the output shaft 3 (i.e., the output), theapparatus according to the present preferred embodiment includes thefirst torque region α1 where the apparatus restricts operation of thepressing assist cams, and the second torque region α2 where theapparatus allows operation of the pressing assist cams. Accordingly, thepresent preferred embodiment is able to prevent sudden, untimely powertransmission caused by accidental operation of the pressing assist camswhen the vehicle including the centrifugal clutch 9 starts to move.

The clutch according to the present preferred embodiment includes thefirst clutch member 4 a connected to the output shaft 3 (i.e., theoutput), and the second clutch member 4 b having the driven clutchplates 7 attached thereto. The pressing assist cams each include anassociated one of the inclined surfaces 4 aa of the first clutch member4 a and an associated one of the inclined surfaces 5 a of the pressuremember 5 that face each other. Accordingly, with the first clutch member4 a and the pressure member 5, the present preferred embodiment enablesoperation of the pressing assist cams.

In the first torque region α1, the first clutch member 4 a and thepressure member 5 are in abutment with each other so as to restrictoperation of the pressing assist cams. In the second torque region α2,the first clutch member 4 a and the pressure member 5 are spaced awayfrom each other so as to allow operation of the pressing assist cams.Accordingly, the present preferred embodiment is able to accurately andsmoothly restrict operation of the pressing assist cams in the firsttorque region α1 and allow operation of the pressing assist cams in thesecond torque region α2.

In the first torque region α1, the centrifugal clutch 9 moves the secondclutch member 4 b without moving the first clutch member 4 a such thatthe first clutch member 4 a and the pressure member 5 are kept inabutment with each other. In the second torque region α2, thecentrifugal clutch 9 moves the second clutch member 4 a and the pressuremember 5 such that the first clutch member 4 a and the pressure member 5are spaced away from each other. Accordingly, the present preferredembodiment is able to restrict operation of the pressing assist cams inthe first torque region α1 and allow operation of the pressing assistcams in the second torque region α2 by operating the centrifugal clutch9.

The present preferred embodiment, in particular, involves making atransition from the first torque region α1 to the second torque regionα2 in the course of operation of the centrifugal clutch 9 (i.e., in thecourse of movement of each weight member 10 from the radially innerposition to the radially outer position). Accordingly, the presentpreferred embodiment is able to continuously and smoothly restrictoperation of the pressing assist cams in the first torque region α1 andallow operation of the pressing assist cams in the second torque regionα2.

Although the present preferred embodiment has been described thus far,the present invention is not limited to this preferred embodiment. Inone example, the apparatus may include no back torque limiter cam (whichincludes the inclined surfaces 4 ab and 5 b). In another example, theapparatus may include no auxiliary clutch plate 17. In still anotherexample, the centrifugal clutch 9 may be in any other form (such as oneincluding a weight member made of a steel ball). The power transmissionapparatuses according to preferred embodiments of the present inventionmay find applications as various multiple-plate clutch type powertransmission apparatuses for, for example, motorcycles, automobiles,three-wheel or four-wheel buggies, or general purpose machines.

During movement of each weight member of a centrifugal clutch from aradially inner position to a radially outer position and a resultingincrease in torque transmitted from an input to an output, a powertransmission apparatus includes a first torque region where theapparatus restricts operation of pressing assist cams and a secondtorque region where the apparatus allows operation of the pressingassist cams. Such a power transmission apparatus may find applicationsinvolving, for example, change(s) in external shape or addition of otherfunction(s).

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

The invention claimed is:
 1. A power transmission apparatus comprising:a clutch housed in a clutch housing that rotates together with an inputthat rotates due to a driving force of an engine of a vehicle, theclutch housing including a plurality of driving clutch plates attachedthereto, the clutch having a plurality of driven clutch plates attachedthereto, the driven clutch plates being alternately arranged with thedriving clutch plates, the clutch being connected to an output that isable to rotate a wheel of the vehicle; a pressure applicator movablebetween an operating position where the driving and driven clutch platesare pressed against each other so as to enable transmission of thedriving force of the engine to the wheel and a non-operating positionwhere a pressing force exerted on the driving and driven clutch platesis released so as to cut off transmission of the driving force of theengine to the wheel; a centrifugal clutch including a mass movable froma radially inner position to a radially outer position due tocentrifugal force produced by rotation of the clutch housing, thecentrifugal clutch being configured to, when the mass is located at theradially outer position, press the driving and driven clutch platesagainst each other so as to enable transmission of the driving force ofthe engine to the wheel and configured to, when the mass is located atthe radially inner position, release the pressing force exerted on thedriving and driven clutch plates so as to cut off transmission of thedriving force of the engine to the wheel; and a clutch spring urging thepressure applicator in a direction in which the driving and drivenclutch plates are to be pressed against each other, wherein at least aportion of the centrifugal clutch overlaps with a portion of the clutchlocated radially outward of the clutch spring as viewed in a radialdirection of the output.
 2. The power transmission apparatus accordingto claim 1, wherein the centrifugal clutch includes a holder holding themass such that the mass is movable between the radially inner positionand the radially outer position, at least a portion of the holderoverlaps with the clutch as viewed in the radial direction of theoutput.
 3. The power transmission apparatus according to claim 2,wherein the clutch includes a boss extending in an axial direction ofthe output and holding the output, at least the portion of the holderoverlaps with the boss as viewed in the radial direction of the output.4. The power transmission apparatus according to claim 1, wherein atleast a portion of the mass overlaps with the clutch as viewed in theradial direction of the output.
 5. The power transmission apparatusaccording to claim 4, wherein the clutch includes a boss extending in anaxial direction of the output and holding the output, at least theportion of the mass overlaps with the boss as viewed in the radialdirection of the output.
 6. The power transmission apparatus accordingto claim 1, wherein the clutch includes a first clutch connected to theoutput, and a second clutch having the driven clutch plates attachedthereto, and at least a portion of the centrifugal clutch overlaps withthe first clutch as viewed in the radial direction of the output.
 7. Thepower transmission apparatus according to claim 6, wherein thecentrifugal clutch includes a press to press the driving and drivenclutch plates against each other by moving in a stacking direction ofthe driving and driven clutch plates in response to movement of the massfrom the radially inner position to the radially outer position, atleast a portion of the press overlaps with the first clutch as viewed inthe radial direction of the output.
 8. The power transmission apparatusaccording to claim 6, wherein the centrifugal clutch includes aspherical body that is partially protruded from an opening of a throughhole defined in the mass and is rollable, at least a portion of thespherical body overlaps with the first clutch as viewed in the radialdirection of the output.
 9. The power transmission apparatus accordingto claim 1, wherein the centrifugal clutch includes: a press to pressthe driving and driven clutch plates against each other in response tomovement of the mass from the radially inner position to the radiallyouter position; and at least a portion of the press overlaps with aportion of the clutch as viewed in the radial direction of the output.10. A power transmission apparatus comprising: a clutch housed in aclutch housing that rotates together with an input that rotates due to adriving force of an engine of a vehicle, the clutch housing including aplurality of driving clutch plates attached thereto, the clutch having aplurality of driven clutch plates attached thereto, the driven clutchplates being alternately arranged with the driving clutch plates, theclutch being connected to an output that is able to rotate a wheel ofthe vehicle; a pressure applicator movable between an operating positionwhere the driving and driven clutch plates are pressed against eachother so as to enable transmission of the driving force of the engine tothe wheel and a non-operating position where a pressing force exerted onthe driving and driven clutch plates is released so as to cut offtransmission of the driving force of the engine to the wheel; and acentrifugal clutch including a mass movable from a radially innerposition to a radially outer position due to centrifugal force producedby rotation of the clutch housing, the centrifugal clutch beingconfigured to, when the mass is located at the radially outer position,press the driving and driven clutch plates against each other so as toenable transmission of the driving force of the engine to the wheel andconfigured to, when the mass is located at the radially inner position,release the pressing force exerted on the driving and driven clutchplates so as to cut off transmission of the driving force of the engineto the wheel; wherein the clutch includes a boss extending in an axialdirection of the output and holding the output, and an outer peripheralwall positioned radially outside the boss and extending in an axialdirection of the output, and at least a portion of the centrifugalclutch overlaps with the outer peripheral wall as viewed in a radialdirection of the output.
 11. The power transmission apparatus accordingto claim 10, wherein the centrifugal clutch includes a press to pressthe driving and driven clutch plates against each other by moving in astacking direction of the driving and driven clutch plates in responseto movement of the mass from the radially inner position to the radiallyouter position, at least a portion of the press overlaps with the outerperipheral wall as viewed in the radial direction of the output.
 12. Thepower transmission apparatus according to claim 10, wherein thecentrifugal clutch includes a spherical body that is partially protrudedfrom an opening of a through hole defined in the mass and is rollable,at least a portion of the spherical body overlaps with the outerperipheral wall as viewed in the radial direction of the output.
 13. Apower transmission apparatus comprising: a clutch housed in a clutchhousing that rotates together with an input that rotates due to adriving force of an engine of a vehicle, the clutch housing including aplurality of driving clutch plates attached thereto, the clutch having aplurality of driven clutch plates attached thereto, the driven clutchplates being alternately arranged with the driving clutch plates, theclutch being connected to an output that is able to rotate a wheel ofthe vehicle; a pressure applicator movable between an operating positionwhere the driving and driven clutch plates are pressed against eachother so as to enable transmission of the driving force of the engine tothe wheel and a non-operating position where a pressing force exerted onthe driving and driven clutch plates is released so as to cut offtransmission of the driving force of the engine to the wheel; acentrifugal clutch including a mass movable from a radially innerposition to a radially outer position due to centrifugal force producedby rotation of the clutch housing, the centrifugal clutch beingconfigured to, when the mass is located at the radially outer position,press the driving and driven clutch plates against each other so as toenable transmission of the driving force of the engine to the wheel andconfigured to, when the mass is located at the radially inner position,release the pressing force exerted on the driving and driven clutchplates so as to cut off transmission of the driving force of the engineto the wheel; and an auxiliary clutch plate secured to the output,wherein at least a portion of the centrifugal clutch overlaps with theauxiliary clutch plate as viewed in a radial direction of the output.14. The power transmission apparatus according to claim 13, wherein thecentrifugal clutch includes a holder holding the mass such that the massis movable between the radially inner position and the radially outerposition, at least a portion of the holder overlaps with the auxiliaryclutch plate as viewed in the radial direction of the output.
 15. Thepower transmission apparatus according to claim 13, wherein at least aportion of the mass overlaps with the auxiliary clutch plate as viewedin the radial direction of the output.
 16. The power transmissionapparatus according to claim 13, wherein the centrifugal clutch includesa spherical body that is partially protruded from an opening of athrough hole defined in the mass and is rollable, at least a portion ofthe spherical body overlaps with the auxiliary clutch plate as viewed inthe radial direction of the output.